When evaluating pen lights, prioritize LED brightness (Lumens) and Color Rendering Index (CRI). For medical use, a warm light (3000K-4000K) with a high CRI (>90) is essential to accurately observe tissue color. For industrial inspections, a cool white light (5000K-6500K) with at least 100-300 lumens is preferred. Additionally, check for pupil gauges or ruler markings silk-screened on the body, and ensure the housing is made of anodized aircraft-grade aluminum for durability and sterilization compatibility.